Tim White is standing with a group of restless men atop a ridge in the Afar desert of Ethiopia. A few of them are pacing back and forth, straining to see if they can spot fragments of beige bone in the reddish-brown rubble below, as eager to start their search as children at an Easter egg hunt. At the bottom of the hill is a 25-foot-long cairn of black rocks erected in the style of an Afar grave, so large it looks like a monument to a fallen hero. And in a way it is. White and his colleagues assembled it to mark the place where they first found traces, in 1994, of “Ardi,” a female who lived 4.4 million years ago. Her skeleton has been described as one of the most important discoveries of the past century, and she is changing basic ideas about how our earliest ancestors looked and moved.

More than 14 years later, White, a wiry 59-year-old paleoanthropologist from the University of California at Berkeley, is here again, on an annual pilgrimage to see if seasonal rains have exposed any new bits of Ardi’s bones or teeth. He often fires up the fossil hunters who work with him by chanting, “Hominid, hominid, hominid! Go! Go! Go!” But he can’t let them go yet. Only a week earlier, an Alisera tribesman had threatened to kill White and two of his Ethiopian colleagues if they returned to these fossil beds near the remote village of Aramis, home of a clan of Alisera nomads. The threat is probably just a bluff, but White doesn’t mess with the Alisera, who are renowned for being territorial and settling disputes with AK-47s. As a precaution, the scientists travel with six Afar regional police officers armed with their own AK-47s.

Arranging this meeting with tribal leaders to negotiate access to the fossil beds has already cost the researchers two precious days out of their five-week field season. “The best- laid plans change every day,” says White, who has also had to deal with poisonous snakes, scorpions, malarial mosquitoes, lions, hyenas, flash floods, dust tornadoes, warring tribesmen and contaminated food and water. “Nothing in the field comes easy.”

As we wait for the Alisera to arrive, White explains that the team returns to this hostile spot year after year because

it’s the only place in the world to yield fossils that span such a long stretch of human evolution, some six million years. In addition to Ardi, a possible direct ancestor, it is possible here to find hominid fossils from as recently as 160,000 years ago—an early Homo sapiens like us—all the way back to Ardipithecus kadabba, one of the earliest known hominids, who lived almost six million years ago. At last count, the Middle Awash project, which takes its name from this patch of the Afar desert and includes 70 scientists from 18 nations, has found 300 specimens from seven different hominid species that lived here one after the other.

Ardi, short for Ardipithecus ramidus, is now the region’s best-known fossil, having made news worldwide this past fall when White and others published a series of papers detailing her skeleton and ancient environment. She is not the oldest member of the extended human family, but she is by far the most complete of the early hominids; most of her skull and teeth as well as extremely rare bones of her pelvis, hands, arms, legs and feet have so far been found.

With sunlight beginning to bleach out the gray-and-beige terrain, we see a cloud of dust on the horizon. Soon two new Toyota Land Cruisers pull up on the promontory, and a half-dozen Alisera men jump out wearing Kufi caps and cotton sarongs, a few cinched up with belts that also hold long, curved daggers. Most of these clan “elders” appear to be younger than 40—few Alisera men seem to survive to old age.